Spaceward Ho!

byPaul GilsteronJanuary 9, 2015

How do you go about creating a straightforward, highly durable design for a spacecraft, one that is readily refuelable and offers manifest advantages for crew comfort and safety? Alex Tolley and Brian McConnell have been asking that question for some time now, coming up with an ingenious solution that could open up large swathes of the Solar System. Alex tells me he is a former computer programmer now serving as a lecturer in biology at the University of California, where he hopes to inspire the next generation of biologists. He’s also a Centauri Dreams regular who was deeply influenced by 2001: A Space Odyssey and the Apollo landings. Below, he fills us in on the details in a narrative that imagines an early trip on such a vessel.

by Alex Tolley

The covered wagon or prairie schooner is one of the iconic images of the 19th century westward migration of the American pioneers. The wagon was simple in construction, very rugged, and repairable. They were powered most often by oxen that lived on the food and water found along the trail. The cost of a wagon, oxen and supplies was about 6 months of family wages.

In 2009 my colleague Brian McConnell and I were thinking about how to open up the exploration of space in an analogous way to the opening up of the American West during the 19th century pioneering era. We were looking for an approach that, like the covered wagon, was affordable, relatively low tech, provided safety in the case of emergencies and the space environment, could “live off the land” for propulsion like oxen, and preferably was reusable so that costs could be amortised over a number of flights.

What follows is a description of the “spacecoach” from the perspective of a new crew member making a first visit to the ship that will be on a Phobos return mission.

Image: ‘Ships of The Plains’ by Samuel Colman.

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Our transfer vehicle docked gently with the Martian Queen airlock. On approach, the Martian Queen resolved into 4 fat sausages, linked end to end. On either side, from bow to stern, were solar PV arrays, partially unfurled. She looked like no spaceship seen since the dawn of the space age.. There was no gleaming metal hull, and she was devoid of all the encrustations of antennae and dishes of those earlier ships. Neither were there any signs of fuel tanks holding liquid cryofuels. Instead, the hull looked dull and somewhat like an old blimp, those non-rigid airships of the early 20th century. The only sign of exterior equipment were those solar PV panels. These were lightweight, moderate performance thin film arrays, extended out on booms to face the sun and drink her rays to power the ship. They looked more like square rigged sails as they fluttered every so gently in the tenuous atmosphere remaining at her orbit.

I knew from the briefing that the Martian Queen needed about 160KW of power, requiring about 800 m2 of arrays at Mars orbit. There was also talk of the next generation “spacecoaches” replacing the PV panels with lightweight rectennas, to convert microwave beams from the orbital transmitters. Most crews didn’t trust that idea yet, but adding a lightweight rectenna was considered a good idea to back up the PVs and also compensate for the lower intensity of sunlight as the newer ships were about to explore Jupiter space. So this was the Martian Queen, the “spacecoach” that would be my home, about to make her 2nd voyage to Phobos.

Following my crew mate Vicki, I passed through the airlock and entered a large space, nearly 60 m3 in volume, shaped like a large cylinder. The interior diameter was about 4.5 meters, about the same as the mothballed Orion I’d seen back at the Cape museum.. But with a length of 10 meters, the volume was 3x larger. The Martian Queen was composed of 4 modules, providing over 200 m3 of full sea level atmosphere pressurized volume, about 2/3rds that of the old Mir space station. Touching the inner skin of the hull it felt flexible, and slightly cool to the touch. A few light taps and the resonant sounds confirmed that there was liquid behind the skin.

Vicki answered my unspoken question about the liquid in the hull. Water was sandwiched between several layers of impermeable Kevlar in the hull. The primary, and ultimately end, use of all the water was for propellant. The spacoach had originally been folded for launch in a standard Falcon 9 fairing. Each module, without any propellant, weighed just 4 tonnes including payload. This was very little and reduced the deadweight mass of the ship. Once in orbit, the interior had been inflated and the hull filled with water. Most of that water had been launched by dumb, low cost boosters, but some was being supplied from extra-terrestrial resources. Supplies from the lunar south pole were becoming increasingly available as Chevron-Petrobras’ Shackleton base was building up mining production. Exploratory vessels were also initiating operations on asteroids, with 24 Themis looking promising with confirmed surface water. In a few decades, it was expected that all water would be supplied from extra-terrestrial sources.

“Why do you put all the water in the hull, rather than in separate tanks?” I asked.

Vicki explained that the water had a number of roles, not just as propellant. The primary reason was radiation protection. The water acted as a good radiation shield, with a halving of the radiation flux with every 18 cm. Starting with about 25 cm of water in the hull, the radiation level inside the module was just 40 percent of that striking the hull. In the event of a major solar flare, the crew could also redirect the water to an interior tube to provide the best radiation shielding for the crew. It looked like that space could get very cozy for the crew, but better than suffering radiation burns.

But it didn’t end there. Micrometeoroids are a rare, but important hazard. The water acted as a shield, absorbing the energy of these grains and preventing penetration inside the hull. The tiny holes in the outer layers quickly heal too. The outer layers of water could be allowed to freeze, trapping a dense forest of fine fibers between the 2 outer fabric layers. This made a strong material, very much like pykrete [1] that offered a stiff outer hull to protect against larger impacts. At Earth’s 1 AU from the sun, reflective foils deployed over the hull allowed passive freezing of the outer layers providing both protection and a large heat sink for the engines.

A noticeable side effect of the hull architecture was the silence. There are no clicks and bangs from thermal heating stresses. Nor did the sunward side of the interior feel noticeably warmer. Thus the water was going to offer very good thermal control of the interior, with pumps in the hull circulating the water providing dynamic thermal control.

Vicki indicated that I should follow her forward to another module. This included the kitchen and dining space. There was a freezer of dried food packages that was being organized by Pieter. Enough for a long trip with a fair variety of meals.

“You seem to have ordered a lot of Boeuf Bourguignon”, joked Pieter.

I wondered when the taste of Boeuf Bourguignon would become rather tiresome after some months. Perhaps more spicy meals like curries would have been more appropriate. I noted that the water supply for rehydrating the food and drinks was connected to the hull too. Of course, I reminded myself, the hull was a huge reservoir of water, effectively inexhaustible are far as the crew was concerned, at least on the outward bound flight.

The facilities were oriented so that “down” was towards the end of the module. This was because during cruise the Martian Queen was going to be rotated, providing some artificial gravity. This made the flight much more comfortable and familiar. We could even eat off regular plates.

Vicki quickly showed me the crew quarters and bathroom in the next module. The inner skin of the hull had been moulded into shapes that could contain water. The baths and showers were also connected to the hull’s water supply. The clean water input was connected to heaters and pumps to the various faucets and shower heads. The grey water from the drains was routed to the main purifier and returned to the hull. I inquired how frequently I could take a shower? Once, twice even three times a week?

“As much as you like”, said Vicki. “There is ample water supply for a single pass through the purifier for all the crew to shower once or twice a day. If the crew is particularly extravagant, even this can be increased with greater recycling. Hygiene is a huge morale booster on these trips.”

The toilet was apparently a composting type, although suitably modified for space. This made sense. The nitrogen and phosphorus was going to be needed for the plants growing in the interior, as well as the Phobos base agricultural areas. Nitrogen and phosphorus were still valuable elements with no rich, off-Earth supplies available. Ducking back into the kitchen space, it was clear that much of the interior was given over to growing plants. They provided the needed psychological connection with Earth, helped recycle the CO2, and freshened the air, removing unpleasant volatiles. The stale, locker room smell of most spaceships was almost absent. Some plants were also growing some fresh foods. I could just imagine the value of a fresh tomato after 6 months of spaceflight!

Image: The Genesis 2 space module. An inflatable habitat launched in 2007 and still operational. A design concept similar to the spacecoach. Credit: Bigelow Aerospace (http://www.bigelowaerospace.com).

Image: Inside Bigelow Aerospace Space Station Alpha mockup. This is similar to the spacecoach basic module before addition of specialized fixtures and fittings. Credit: Bigelow Aerospace.

Pulling ourselves back through the leafy interior of the modules, I looked for the engine compartment in the last module. The engines were not obvious on docking, and I wondered where they were. At the rear of the last module, an airlock was currently open, showing an enclosed space beyond. Inside, Hans, the engineer was taking apart one of the engines. He was removing a metal liner from the engine and replacing it with a fresh one. He handed the old one to me and said “carbon deposits”.

I looked closely and saw what he was talking about. Carbon deposition from contaminants in the water supply could build up in the engines, reducing performance. The engines were not much more complex than microwave ovens, although they were fitted with electric grids to further accelerate the microwave heated water plasma.

The exhaust exited via the rear, when the bay doors were opened. Now they were closed, allowing the shirt sleeve repair of the engines. I asked how frequent engine repairs were. Hans informed me that an engine needed some rework after 3-6 hours of operation. The microwave electrothermal engine performance had an Isp of about 800s, although the secondary electric grids could double that by drawing on reserve energy from the solar arrays. Vicki thanked Hans and we drifted back to the main module.

I was a little surprised at the lack of windows, but pleased that there were many flat screens where windows should have been. I looked “out” and saw that I had missed the vernier and maneuvering jets on the hull.

“How are these powered?” I asked Vicki.

“Hydrogen Peroxide, H2O2” she replied.

“Where’s the fuel?”.

“There isn’t any yet. It’s made during the flight. Some of the water in the hull is tapped off, run through that off-the-shelf, standard unit over there. We store the peroxide in hull pockets to wait for the next use. The peroxide engines aren’t very efficient, having an Isp of about 160s, but they provide higher thrust than the main engines and can be used to boost the ship for a faster departure, or land the ship on low gravity worlds with orbital delta-Vs of 0.5 km/s or less. The peroxide has other uses too. It can be decomposed to provide oxygen [3] more quickly than the main ESS electrolyzers, act as an energy store for emergency power [4] and finally as an excellent bactericide to keep the interior clean and remove the bacterial slimes and molds that grow on the inner skin, often in difficult to reach spaces. And before you ask, yes, we have rotating cleaning duties on the Martian Queen.”

So the water in the hull fulfilled a range of uses, before being finally consumed as propellant. Major uses included bathing, direct consumption, rehydrating food, growing plants and, of course, the main oxygen supply. It was converted to peroxide for the high thrust engines, for energy storage and for another emergency O2 supply.

“Vicki, a quick mental calculation seems to come up short on the water requirement for the flight. Is what I see all that is needed?”

Vicki smiled: “The impact of using water as propellant on performance is significant. The total water budget for the trip is about 4 times the total mass of the ship and payload, compared to about 14 times for a conventional liquid hydrogen and LOX chemical rocket, primarily because of the higher Isp of the electrothermal engines. But the low hull mass and reduced consumables payload reduces the main mass of the the Martian Queen allowing a much smaller, more efficient spaceship. She is also a lot roomier, more comfortable and much safer. An Apollo 13 type accident would not be survivable in a conventional ship, but we have very large reserves of consumables and oxygen for the crew to survive until a rescue or the return trajectory was complete. In addition, even without water supplies at Phobos, the baseline mission cost to Phobos and return is on the order of a $100m dollars. That is why your institution can afford to pay for your slot on this mission. Reusability of the Martian Queen for multiple missions, fresh water at Phobos, and better performing solar panels and electric engines will eventually reduce that cost perhaps another order of magnitude.”

I pondered that for a moment. While not a cheap solution for interplanetary travel, it put the cost well within the realm of the super-rich and wealthy institutions. A mere decade earlier, a simple lunar flyby and return in an adapted Soyuz craft was priced at around $100m per passenger by Space Adventures. Spaceflight was definitely getting cheaper and safer.

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If interplanetary travel is initially based around the design concepts of water propellant craft, then the economics and infrastructure requirements will be dependent on available supplies of water already in space at suitable locations for fuel dumps. Bodies that may harbor economically useful quantities of accessible water include the moon (shadowed polar regions), water rich asteroids and dead comets. A tantalizing possibility is Ceres, that Dawn is expected to rendezvous with this year (2015). Ceres is expected to have prodigious quantities of frozen water, possibly even a subsurface ocean. A mining operation to extract pure water from the brew of ice and chemicals might offer the opportunity to open up the inner solar solar system. Once at Jupiter, the icy moons offer an almost inexhaustible supply of water.

Christmas comes late this year…No use crying over the lost opportunity of not building the enormous von Braun moon rocket of 1954-55…Bigalow’s Genesis 2 is key to the planets…The inflatable space wheel of 1954-55 comes again in the form of a large inflatable room…Better late than never…Sagan said to take little baby steps anyway…spaceward ho…

Thanks Alex. That was an excellent way to get those ideas out there, bravo.
I’d love to get aboard a Bigelow inflatable in orbit; sure looks spacious (been daydreaming that ever since photographing a pass of Genesis a few yrs back from my inlaw’s back garden here in Newport, UK)

One question about your engines… are they along the lines of VASIMIR and other SEP designs?

Great job on this post, Alex! I rather enjoyed reading it. It’d be great if NASA heavily invested in developing and testing this kind of spaceship with their HSF program. Or maybe SpaceX and affiliates will get there on their own!

Bigelow is at true pioneer, however the one person that he could team up
with to create the future faster , Elon Musk, do not see eye to eye regarding the capabilities of Space X Dragon capsule for manned flight, Bigelow has hitched his wagon to Boeing’s CSX 100. I don’t know if it’s loyalty to known a NASA contractor (Bigelow being a NASA alum), Or maybe Ego or distrust but it is an unfortunate state of affairs as Bigelow’s vision of a Space Hotel, is something I want to experience. The more manned vehicles developed the better the chances the prices will drop to travel there.
On the other hand:
Good grief, growing plants in a limited space and having to use precious
water to keep them alive? I predict improvement in freeze drying tech will
make any ‘space farmers’ out there uncompetitive for a long time.
The volume of 1 CU meter of freeze dried food, combined with 6 CU meter of water (on average) will give you the capacity to feed 6 persons for 1 year.
I will leave it agro-enthusiasts to calculate how much water and chemicals
and just plain space would be required to get a varied diet such as freeze dried products can. (even vegetable gardens take work and space, although for make-work to relieve boredom it’s not bad)

Hi, a bit of info about the engines. We specifically looked at microwave electrothermal engines. You can think of them as a poor man’s ion drive. They’re basically microwave ovens, except instead of slowly heating a large mass, they dump a few kilowatts into a small stream of water vapor, which gets superheated to produce thrust. They generate a specific impulse of about 800 seconds, double or triple a chemical rocket, but a lot less than an ion drive or vasimr.

What’s interesting, especially for solar electric propulsion, where the size of the power plant is an issue, is that too much specific impulse is a bad thing. Very high Isp engines use less propellant, but they also require huge amounts of power to generate useful thrust for pushing people around in a reasonable time frame.

It turns out that MET engines are just efficient enough to realize big cost savings, and fit more or less in the efficiency sweet spot for efficiency / power requirements.

I would be worried about using H2O2 in concentrated form as it can decompose violently. We might be better off using CO and Oxygen as a propellant, both are readily available as free or near free materials in the moons polar ices or regolith. It would be easier than breaking up hydrogen and oxygen and it makes a fairly good rocket ISP.

As for reusability we could use the Orion modules by joining them via connection modules to form a greater complex or we could use the organics on the moon to form plastic shields that are just added to outer skins of the Orion modules allowing greater distances to be travelled more safely. Getting back to the moon is a great priority if we are to make it as a space faring species.

As for inflatable modules I just love the idea, we can inflate a torus module for instance made of rubber easily and then spray aluminium/alloys onto the outer skin to form the basic shape, the ideas are endless.

We had envisioned generating and storing H2O2 in dilute form (~10% conc), where it is completely safe. This reservoir could later be vacuum distilled to higher concentrations if needed for propulsion. If not needed for propulsion, there is a _huge_ bonus feature, as the dilute H2O2 can be decomposed to generate oxygen for life support. Plus you don’t have to worry about algal or bacterial contamination.

Very interesting and insightful. If I had to nitpick something would be that microwave beaming only makes sense from orbit to Earth’s surface, since it is barely absorbed by the atmosphere. On interplanetary beamed power, you would prefer short wavelengths to minimize diffraction. The shorter you can provide at a given power envelope, the better

@Mark – the electrothermal engines do produce ions, but very duifferently from VASIMR. The working fluid, in this case water, is just heated by microwaves until they dissociate into a plasma. Doing that in a pressure chamber results in a host exhaust just like a rocket engine. That gets you to 800+s depending on design. Since you now have ions, they can be accelerated further just like an ion engine, although with more difficulty due to the reactive nature of the ions. Arcjets are probably the most similar.

We’re agnostic about the engines. 1000-1500s seems to be a “sweet spot” for run to Mars, but whatever works best. You could use a linear accelerator throwing ice rather like O’Neill’s ideas, but in his case throwing asteroidal rubble.

Brian has provided the best answers to your question, I think.

@Rob Flores – you are absolutely correct about the volume of water tied up in agriculture. I included it because the anount of water was so huge that it gave the reader a better sense of how much water was available before being used as propellant. In practice I agree that rehydrating dried food makes the most practical sense. However even I would like to bring a few tomato and Serrano pepper plants along just for esthetics and potential fresh food and spices. If I was outfitting the ship, I would make sure that the showers were top of the line. :)

I too hope Bigelow can get a commercial win with his approach. He seems to be making progress, but he really needs a commercial, full orbital, tourist service to really create a demand for his space “hotels”.

@James – I have pictures of those Goodyear rubber inflatable station mockups from the 1950’s. The idea seems sound, although I can see why it made more sense to reuse the spent stages, as Skylab did. Now that we are back to building modules again, time to revise these ideas of inflatable structures, and hopefully with at least partial gravity. My guess is that tourists will want gravity to make their stay comfortable and as familiar as possible, while still having access to zero g.

Amplifying Alex’s point about engines a bit. We view them as an interchangeable component. Anything that can process water/water vapor as reaction mass is a candidate. We focused on METs because they’re fairly well along in development, simple in design, and can be scaled using arrays. As better engines come along (in terms of specific impulse, thrust/weight, thrust/power, etc), they can be flown up and installed (engine weight is tiny relative to overall ship mass, so upgrading is cheap).

The most important point though is that > 80% of the ship is water, by mass, when it starts its trip. So if the empty weight is on the order of 20 tons, you’ll start off with several times as much water. In a conventional ship the fuel is dead weight. In a spacecoach almost everything is propellant (even consumables, since waste water can ultimately be sent to the engines). That completely changes things, since you no longer need to trade crew consumables, comfort or safety margin against propellant.

Thankyou Brian and Alex for the helpful replies… the MET engines sound perfect as does the H2O2 storage you outlined. I’ve just finished the .pdf link after reading up on MET so thanks for the pointers there, very interesting engines.

Maybe things will progress more rapidly for the space-hotel companies after the future ISS inflatable tests generate more public awareness? That could be a gateway to a future so eloquently outlined above.

@CharlesJQuarra – Correct me if I’m wrong, but didn’t this get addressed when discussing beamed sails? I thought that phased microwave arrays were a good way to beam power, even over solar system distances, particularly from an energy efficiency standpoint. Of course the expert is Jim Benford regarding this.

Alternatively, as you suggest,, lasers might be a good solution, especially as the ship already has solar arrays.

One other item that should probably be mentioned is the solar power array’s power density. A key aspect of the spacecoach is that most system elements will be replaceable, including the engines and solar arrays (with a rigid beam, T-bar or kite-lite main hull, the arrays will not need a rigid truss, but can be lashed like sails, at great mass savings).

The key factors in power plant output will be: a) solar cell efficiency (40% is definitely achievable, possibly 50% with multilayer cells), and b) power density (watts per kilogram). The latter factor is what will determine how far out spacecoaches can travel on solar power. If you can increase power density by a factor of four, you’ll be able to generate the same peak power, and hence acceleration, twice as far out. So we expect that optimizing for power density will be one of the areas of ongoing improvement.

We worked out that early generation ships will be able to travel anywhere from Mercury to the Asteroid Belt (and Ceres). That ought to keep people plenty busy while engines and powerplants are gradually improved to enable trips even further afield.

“Ducking back into the kitchen space, it was clear that much of the interior was given over to growing plants.”

I’d think much of the interior throughout the ship. Probably with LED lighting specifically tailored to maximize photosynthetic efficiency. Plants basically don’t use the center of the spectrum, which is why they look green. For maximum efficiency, you want mostly red light, with a little blue, but the ideal mix varies between different points in plant development. Optimized LED lighting is enormously more efficient at driving photosynthesis than sunlight.

Of course, from a transit standpoint, you’d probably be better off just storing dehydrated food, which the passengers would neatly transform into H2O and CO2 over the course of the trip, that could be fed into the engines. And just use some plants for flavorings and psychological benefits.

Containerized urban agriculture tech can most likely be adapted to the spacecoach. We assume that in the first missions, agriculture will be mostly experimental and primarily for psychological benefit (e.g. growing spices, flowering plants, etc).

That said, the CO2 and organics humans emit will have to go somewhere, and once we figure out what plants and algae do well there, better to convert them into plants and/or biofuel (which if not used for propulsion, can be used as feedstock for complex organics, plastics, etc). At the very least, they can be left at the destination for use in future missions. As Robert Zubrin pointed out once, in space, shit will be worth more than gold.

What’s needed are biosphere type craft beginning with the simplest organisms and becoming more and more complex as we improve. Once a craft with, say, fungi and algae plus one cell and simply multicellular animals is shown to function in Earth orbit it should do a loop around the moon and return. Eventually we might have self-sustaining biospheres making a loop around Mars and returning — before we try it with really complex life, of course.

@DCM: It is not necessarily true that the simplest organisms are also the simplest to maintain. From “experimentation” on my (multicellular) houseplants I know that some rather large ones can thrive on a liter of tap water a week, plus light from a window and regular air. Nothing else for over 20 years, at least not provided by me. Same pot of soil as I bought them in. That is pretty darn good in the self-sufficiency department.

In general, I believe the difficulty of maintaining stable biospheres is widely overestimated, although I do not want to suggest it is trivial, either.

‘Eventually we might have self-sustaining biospheres making a loop around Mars and returning — before we try it with really complex life, of course.’

I was thinking along the same lines with a twist, if we had biospheres that made loops say around the moon and back around the earth all the time could we not just catch up with these well protected habitats and use them as transfer hotels. We could have a few around the solar system and all we would need to do is hop aboard them via a faster lighter craft.

@Brett Bellmore

‘If we had fusion engines, anyway.’

A fusion engine in space is a lot easier to ignite and keep going than heavy confinement equipment needed to contain the plasma for power generation.

@AlexTolley, If I remember correctly (should check the paper to confirm) Benford’s proposal relied on a rectenna array at Earth’s surface, so the choice of wavelength still makes sense to minimise atmospheric absorption – But if all the processes of solar energy capture and beaming are in space, you are free to choose better wavelengths

For instance, solar-pumped lasers have high theoretical conversion efficiencies, and they would sit ideally on the peak of sun’s thermal spectrum, somewhere about 700 nanometers. The reduction in beam divergence over a similar microwave beam are several orders of magnitude

From IO9 website
“About 300 to 500 plants would produce the right amount of oxygen, but it’s much harder to estimate the amount of carbon dioxide the plants absorb, especially if every time a person breathes out, they inhibit oxygen production. To be safe, don’t get into an airlock without bringing about seven hundred potted plants with you.”

Assuming each pots is a square shape of 6in (why makes things harder out there)
With 700 plants You will need an area about 13.25 sq feet square. but wait you will need to tend this garden with care, so you will need open passages to operate there. A passage 2 feet wide and placed at intervals will make your room about 756 sq feet something like a 44×17 room including a bit extra room around the edges. So this had to bee tended, watered, and fed,
for ONE astronaut to remain alive. I suppose you could use shelving if
you wired for UV lighting in the lower section. This would halve your needs
to 22×17. Better, but I hope your don’t get any leaks, causing a short, and then a fire.
I don’t think this is even good as back-up recycling system, you would still be better off with emergency scrubbers which can be recycled and LOX, tanks, for that.

This is not only an innovative use of technology, it’s also emotionally uplifting. Getting the public emotionally involved in space travel is perhaps the most important puzzle piece to fit. I hope the ideas presented gain traction relatively soon. NASA needs to think beyond Orion and Space Launch System and include human habitation in it’s near-term goals.

The practical and simple concepts you present rise to the visionary level expounded by Robert Zubrin in his books “The Case for Mars” and “Entering Space.” Unfortunately the ideas in Zubrin’s books have languished without NASA action for 18 and 15 years respectively, despite Zubrin’s strong advocacy directly to the space agency.

Why is the vision of the visionary often ignored for so long? Perhaps, as demonstrated by the weak “solutions” to global climate change that have been feebly enacted, humanity is inherently wed to inertia.

Alex: You needn’t guess. You know. Trust your judgement. I hope y0u’ll consider a ring of separate inflatables, revolving for gravity about a central hub, with a corridor connecting, say, all 36 inflatable rooms. The docking hub would be our door to the stars. Musk logically just follows whichever river of money is flowing his way. It has to be this way. Bigalow remains in the race to the moon and planets.
Spaceward ho!

We’ve considered a range of configurations. We’re working on a book which covers the options in detail. One of the next steps is a set of design competitions. One competition will focus on vacuum chamber tested electrothermal engine prototypes. One will focus on solar array designs to maximize power density. And lastly, a ship design and system integration design to come up with winning spacecoach candidates. So stay tuned.

The reduction in beam divergence over a similar microwave beam are several orders of magnitude

Only if you assume similar aperture. Phased array microwave apertures can be made orders of magnitude larger than those of lasers, though, with large structures or formation flying. I think that for this reason microwaves are still in the running, despite the drawbacks you mention.

This is a really interesting concept. At the moment I can easily see this or a variation as a prototype of the MCT.

One point. I suppose it is silly but on reading the article I keep seeing the water in the hull eventually used as propellent. To my mind this is seeing my radiation shield going away and I die at the end of the trip. I presume that what is really happening is that the water used as propellent on top of the water used for the radiation shielding and other uses.

Re the plants everyone is commenting about: like showers and housekeeping, I would suspect that gardening duty would also serve as a welcome psychological distraction during the trip.

Re: radiation shielding. This is all about the cumulative radiation dose. It is true that you draw water down over the course of the trip. The thing is you start out with a huge amount of water, enough to inflate a meters thick bladder when you need it (solar flare), enough to reduce radiation flux 10,000 fold. Toward the end of the trip, you won’t have that sort of margin to work with, which just means you’ll need to be smart about where you put the water and how you orient the ship to get maximum protection during an emergency. Mission planners will always budget extra water for contingencies like an Apollo 13 scenario, so even towards the end of a trip you should always have enough water that you can fashion a temporary shield with several halving distances. You also have plenty of electrical power you can probably also use a magnetic field to divert charged particles, but we haven’t looked into that.

Stored pure liquid hydrogen peroxide is definitely dangerous in liquid form, but is it so when frozen?

How can you buy water from the Moon when there are bases there? If any base has thoughts of ever starting to build a complete ecosystem there to support a thousand people, buying Chevron-Petrobras’ Shackleton base water at ten times the price the Martian Queen, would be a trivial cost of that future investment. If a hundred or so different (national) Moon bases had similar thoughts, (or worse still, used the latent heat of freezing water to protect them on the long lunar night) their minimum water requirements would quickly reach 100% of all the cheap polar water.Surely the Moon is far more likely to be importing water/hydrogen?

@Rob, hydrogen peroxide can be easily stored at lower concentrations (up to 30% no problem), so most likely you’d store it in dilute form, and vacuum distill it to high concentration only when needed. Meanwhile the dilute form can be used as a disinfectant, or treating wastewater, and a simple means of generating oxygen, all good things to have on a long duration trip.

I still think you would want to compartmentize the Water reservoir in
Each module esp on long duration mission far from help. I am imagining getting hit by something greater than a sand grain. Assume you get hit by 3mm pebble, moving at 30,000Km/hr
even if your kevlar layers can prevent a total breach, I am thinking the
power release on the water mass and subsequent wavefrong might cause any seams to rupture (probably on opposite side) on the hull.

@Rob – those are exactly the sort of issues that need to be considered in a workable design.

The idea is still in its early stages, and we think it shows promise as a way to inexpensively travel in space using technology that is pretty much available today. The broad outlines look promising and need work to ensure that there are no show stoppers and that the advantages we see are still in place with better designs.

@Michael – to add to Brian’s comment – it is important to think about the possible adaptive responses. E.g. One module would retain a full shield as long as possible to maintain protection. Orientation is another. Smaller volumes with thicker shields can be filled in the case of a solar flare. And so on. These tactics enhance the radiation shielding effectiveness beyond naive assumptions. I hope this clarifies our thoughts.

@Rob, compartmentalizing the reservoir makes total sense, and will not result in much of a mass penalty. Alex and I also looked at using pykrete (water frozen into fibrous material) for debris protection. It would be interesting to test a pykrete + kevlar combo so see just how much it can stand up to (it’s certainly way better than a tin can).

The ship configuration is also important. If it is designed so there are multiple paths between any two compartments, a compromised module can be sealed off. We haven’t really gotten into that yet, but there should be some economically good solutions that also offer very good options in the event of a debris strike rendering a module uninhabitable.

I hope Brian/Alex you will test industrial hemp fibres as a potential water holding medium as the material has many other uses such food, clothing, ambiance and building materials (Ford’s axe proof car!) in many ways a unique material. It has many advantages over artificial materials just the right soil, water, sunlight and of course air pressure.

Inflatable habitats do seem to be the way to go, reducing the needed launch mass by as much as several orders of magnitude. The water filled walls also fit right into this. However, wouldn’t it be better to build the ship in a spherical shape? This gives maximum volume for surface area, allowing the ‘shell’ of water to be thicker while reducing the total mass needed. Also, as the diameter will be greater than a cylinder, the whole caboodle can be rotated to provide simulated gravity, perhaps a third of a gee at the ‘equator’ of the ship. The inflatable structure should mean that the hull can still be launched on a conventionally sized commercial booster, deflated to give a small diameter. The centripetal force of rotation could also be used to extend the solar PV ‘sails’. And the water itself may not be too costly, if the ice on the moons of Mars is mined and shipped with low cost methods, although there would have to be a fair amount needed to justify initial cost.

‘Also, as the diameter will be greater than a cylinder, the whole caboodle can be rotated to provide simulated gravity, perhaps a third of a gee at the ‘equator’ of the ship.’

Unless the sphere has some high rigidity and/or air pressure it would tend to collapse at the slower rotating parts when generating artificial gravity and all the water will move to the equator. Truly huge structures can be built in space, inflatable modules would only be stepping stones.

There are lots of possible configurations but keep in mind that the power source is a solar array, so you need a ship that has at least a football field sized array the array can be tied to, and provides a useful habitable area for the crew.

We looked at ring shaped ships with the arrays lashed inward. Those will be interesting, but inflatable hab tech isn’t quite there in terms of mass per unit of volume. So we ended up with the kite like arrangement for now.

The whole point is that as things improve gradually, you can re-use and re-assemble components.

Im a little late to this thread, but I fear that it will be hard for us to live in space for longer periods of time.. First you need to find the soon rare people that dont are obese, have prediabetes/ diabetes or other modern food related diseases.. These people will need real fresh real food, not some freeze dried unnatural food thats gone through an unnatural process? – But Im not knowledgeable in the freezy drying area..
How are going to get real meat into space or hens so we can at least eat nutritious eggs? We need more studies on animals in space, and I think hens would be a good start, there is not much more needed to fill the daily nutrition intake if you have eggs. Maybe you could just hard boil the eggs before you send them up.. But that would need a working infrastructure for earth to orbit transport. Not the behemoth and fragile rockets we have today.. I dont know, but I dont believe we have the knowledge about our bodies yet to start living totally on factory-food. Just look what the low-fat /high carbohydrate recommendation in the 70’s has done to the american people in the last 40 years.. It did NOT make them healthier, thats for sure.. And still people believe in these recommendations.. Do I really need to mention the cholesterol-myth that still is floating around even amongst those how should have the knowledge and interest to learn the latest science?
Well, Im just babbling to much. I think food will be a bigger hurdle for longer periods in space then we realise today. Excuse my bad English, I hope you get my point. Thanks.

The idea of using water and inflatables is not new. I remember reading an SF book forty years ago about a rush to the asteroids using such a technology. I wish I could remember the book’s name. Still, this is the kind of thinking we need. This is technology that is here now and can be adapted as new tech comes on line. I hope a space entrepreneur picks it up and runs with it.

Joseph, I haven’t come across the book you mention,but one with a similar premise was Gallagher ‘s Glacier, by Walt and Leigh Richmond. In that one the hero made a spaceship by finding a big chunk of orbiting ice and fitting an engine to it. He made tunnels and cavities inside a as he needed them, shaping and reshaping the ship as needed.

Closer to the Spacecoach is the space station in Steven Gould’s novel Exo. In that novel, an individual who wants to build a space station on the cheap makes a pair of concentric inflatable spheres, with a layer of water mixed with pulp. This fills the function of radiation shielding and meteor protection as outlined here.

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